Why does my phone sound like a siren is going off?

I have an iPhone 6 Plus and all of a sudden a siren noise started and will not discontinue even if I shut the phone off. HELP!! It is very loud and the volume is not controlled by the ringer increase/decrease button.

Someone has logged into the iCloud website with the iCloud credentials associated with the device and has activated the lost phone signal, so to speak. It's usually associated with a list or stolen device. If your situation does not fall into this category, make a Genius Bar appointment with the nearest Apple Store.